Holothuria mammata is one of the most valuable species of sea cucumber, as well as one of the main target species harvested in the Mediterranean and NE-Atlantic regions. This study aims to describe the reproductive cycle of H. mammata in a coastal area of southwest Portugal. Monthly samplings were carried out for 19 months, with the concomitant collection of environmental data and biometric data. H. mammata had a sex ratio of 1:1.2 (male:female) and a size at first maturity of 142 mm for males and 167 mm for females. The gonadosomatic index (GI) peaked between April and May for both sexes. Gonad development started when days had a shorter photoperiod (9 to 13 h of sunlight) and lower seawater temperature (15 °C) and chlorophyll-a concentrations. The development of new studies to increase the biological and ecological knowledge of the populations of H. mammata is essential to create conditions for the domestication of broodstock in captivity, to allow the development of fishing regulations based on informed decisions and to create precise measures for the conservation of ecosystems.

The demand for sea cucumbers from the NE-Atlantic is increasing as a consequence of the depletion of stocks that historically supplied Asian markets. Some species from the NE-Atlantic are already showing overfishing signs due to either inefficient policies or their total absence, and they do not offer any margins of sustainability to sea cucumber fisheries. The current lack of knowledge of biological and ecological traits has led to ineffective protection of those species. Holothuria arguinensis is a good example of a temperate sea cucumber species suffering from the effects of unregulated and illegal harvest. This study aims to determine the temporal and spatial distribution patterns of H. arguinensis, assessing density and size-class structure in a NE-Atlantic population, in SW Portugal, as a function of environmental conditions. Sampling was done every 1.5 months at 6 stations defined according to habitat heterogeneity, following the coastline from the mouth of the Sado estuary to marine coastal areas. At each station, there were two habitats (rocky substrate and sandy areas) sampled through SCUBA diving along 30-m random transects, each with three replicates. Individuals were counted and measured (total length) and individual behavioural parameters, such as sheltering and settlement surface, were recorded. Water column parameters, sediment composition, and substrate cover were also recorded. Generalised Linear Models were fit to the data, modelling density and size distribution of the species with respect to environmental conditions. The results indicate that the distribution of H. arguinensis was mainly determined by its ability to deal with physical environmental stressors (current), rather than the chemical properties of the water column or sediment composition. The hydrodynamic features of the region exert the greatest impact on this species’ distribution and help shape habitat usage by different size class groups.

The ever-growing demand for sea cucumbers is a threat to these echinoderms and their habitats; however, a way to relieve stock pressure lies in meeting demand through aquaculture. As such, this study aimed to improve the growth and survival percentage of Holothuria arguinensis during larval development, settlement and juvenile growth. Three diets of microalgae (Diet RbPt = Rhodomonas baltica + Phaeodactylum tricornutum; Diet RbSm = Rhodomonas baltica + Skeletonema marinoi; Diet RbCc = Rhodomonas baltica + Chaetoceros calcitrans) were administered daily to the larval phase. Additionally, three substrates were supplied (pvc rolls, wavy pvc plaques and tile) at three different depths in order to determine the most favourable for settlement. Lastly, the potential benefits on growth and survival from introducing macroalgae (Sacchoriza polyschides) in the juvenile diet were assessed over a period of 6 months. Despite larvae under diet RbPt having presented a larger mean width, H. arguinensis fed with Diet RbCc presented a higher settlement survival at the end of the trial. No preferences were noted among the tested substrates, regardless of diet. Lastly, juveniles with added S. polyschides in their diet showed increased growth in mean weight and length when compared to individuals that were not fed with this seaweed.

Sea cucumbers, integral components of benthic ecosystems, have become subjects of scientific scrutiny owing to their intricate morphology and ecological importance. Due to increasing demand, several species of these echinoderms have become overexploited. As a consequence, NE-Atlantic species became new targets for the international markets. There is a vital need for comprehensive biological data to establish and enhance holothurian fisheries management. In the absence of such data, there is a risk of ineffective fisheries regulations, particularly for susceptible commercial species in the NE-Atlantic, which could lead to overexploitation. Establishing effective fisheries regulations requires a foundation of fundamental biological information, such as growth rates from target populations. This work aims to determine the growth parameters for three commercial sea cucumber species from the NE-Atlantic, in a marine protected area, Holothuria mammata, H. forskali and H. arguinensis. This presents a challenge, as sea cucumbers lack significant calcified structures for age determination, and assessing size is complicated because of their body size plasticity. Thus, a von Bertalanffy model was fitted to length-frequency data using the ELEFAN method with the simulated annealing procedure. Underwater length measurements of the three sea cucumber species, in a relaxed state, were systematically recorded over a span of 18 months, with measurements taken every 1.5 months. The results provide the growth parameters of each species. Models considering seasonal growth were better fits to the data. The three species had different growth rates and periods of no growth coincided with the reproductive season. Mortality was lower for the species growing slower. This is crucial information to support decision-making processes regarding stock management, such as setting limits to fisheries considering stock condition associated with environmental variability.

Sea cucumbers (Class Holothuroidea) are echinoderms with an extremely important ecological role. They dwell on the benthic zone being either deposit or suspension feeders, feeding on tiny particles like algae, small aquatic animals, or organic matter. The holothurians reduce the organic load of the ecosystems while excreting inorganic nitrogen and phosphorus, playing major roles in nutrient recycling while enhancing the productivity of benthic biota. Apart from nutrients redistribution due to ingestion and release of fecal matter and by their locomotion across the sea bed, sea cucumbers also play an active role on ocean acidification buffering. Some species can increase water alkalinity and dissolved inorganic carbon through their digestive processes and release of ammonia, thereby facilitating calcification by organisms such as corals and increasing the seawater buffering capacity. Commercial exploitation of sea cucumbers provides sources of income to millions of people, mostly in Asian countries. Global assessments reveal that overfishing is a big concern and is leading to an accentuated decline of several sea cucumber populations, generating an increasing number of threatened species. Overfishing is also likely to cause a reduction in value and promote a change in the harvesting regions. The distribution of commercially exploited sea cucumbers is of special interest because their natural abundance can be greatly altered by fishing. This study aims to understand how environmental variables and geomorphological characteristics may influence the distribution and abundance of three sympatric sea cucumbers species. While most of the ecology of sea cucumbers of the Holothuriida Order is well known, north-eastern Atlantic holothurian species lack information. Environmental variables were analyzed to see how they can affect the distribution of Holothuria forskali, Holothuria tubulosa and Holothuria arguinensis in the Sado estuary and its adjacent coastal area, three sea cucumber species of commercial relevance. The sampling campaign took place monthly, during the winter and spring seasons, and comprised a total of 16 sampling stations for this study of which, 10 were in the Sado estuary, west Portuguese coast, and 6 in its adjacent coastal area. Environmental biotic and abiotic parameters were collected using a multiparameter EXO2 sonde. Geomorphological features with potential influence over sea cucumbers’ distribution were also considered, such as local current exposure and presence of shelter. In addition, nutrients and phytoplankton pigments were quantified in the water column, through HPLC, as well as sediment total organic matter (TOM) and granulometry. In each sampling station two habitats were defined, sand and rock, and three replicates were done in each, parallel to the coast, at the same depth stratum. Every transect was thirty (30) meters long and three (3) meters wide covering an area of about 90 square meters. Sampling was carried out by scuba diving and sea cucumbers were identified to the species level, counted and total length was measured in situ along each transect. Throughout the sampling period, a total of 1541 sea cucumbers were sampled of which 59 were H. arguinensis (3,8%), 545 H. tubulosa (35,4%) and 937 H. forskali (60,8%). To test if there were differences, in relative abundances of sea cucumbers, between the sampled months, a two tailed Wilcoxon test was performed. Paired samples included same stations for each month. The Wilcoxon test was done for the total of the three species and for each one of them. To explore the differences of relative abundance of sea cucumbers between habitats (rock or sand bottom), distribution (estuary or coast) and sampling stations, analyses of variance (PERMANOVA) and ordination methods (PCO - Principal Coordinate Analysis) were conducted. All analyses were performed with a significance level of 0.05. No temporal (monthly) variations were observed in relative abundances, either for the total of the three sea cucumber species or for each species. However, the spatial analysis, revealed that the abundance of each of the three sea cucumber species was higher in coastal stations, negatively correlated with turbidity, temperature and sediment grain size, and there was a preference for rocky habitats within the marine environment. Additional pair-wise tests and the PCO analysis suggested that the site’s exposure to ocean currents (site geomorphology) and the absence of shelter (rock crevices and depressions and closed angles between substrates) have a negative impact on the abundance of these sea cucumbers. The results of this study show that these three species have a preference for coastal areas and rocky habitats providing shelter, as opposed to open areas or flat rocky substrates. Moreover, the spatial patterns of distribution and abundance appear to be relatively stable and none of the three species occurs under estuarine conditions. Apart from helping with fisheries strategic planning, these results highlight the importance of rocky reefs in the Atlantic distribution area of these, ecological and commercially valuable, sea cucumber species.